To the OP, if you haven't seen my thread yet, here's my used oil analysis from Blackstone Labs. I will be extending my 6.8K avg oil change interval up to 10K miles (w/ up to 2 track days) from now on. I'm sure I run my car harder than most tuned cars here, so take it for what it's worth. Every car performs differently, so I recommend doing your own UOI to fit your car and your driving style.

Send an oil sample to Blackstone Labs your next oil change and you can stop wondering. They'll analyze your used oil sample and tell you how your motor is wearing and how much longer you could have gone on that oil (if any) based on the amount of additives left and how broken down the oil is. Chances are you'll be able to easily go 7-10k miles or longer on long life synthetic even if you drive hard.